1. Field of the Invention
The present invention relates to an apparatus and method for communicating a signal in an asymmetric digital subscriber line (ADSL) network by using dual link discrete multi-tone, more particularly to an apparatus and method for transmitting and receiving a signal in an ADSL network by using a dual link discrete multi-tone in which low rate signals e.g., signals of a plain old telephone service (POTS) and an integrated services digital network (ISDN) are transmitted through a low pass link and high rate signals e.g., a packet and a superframe are transmitted through a high pass link, thereby controlling a traffic.
2. Description of the Prior Art
In general, the ADSL is an asymmetric digital subscriber line having a upstream transmission bandwidth from a subscribe to a telephone central office and a downstream transmission bandwidth from the telephone central office to the subscriber wherein the upstream and the downstream bandwidths are discriminated each other. As for the ADSL, the downstream signal bandwidth is larger than the upstream bandwidth and a high speed signal of about 6 Mbps can be transmitted to the subscriber separated from the telephone central office within a distance of about 4 km.
FIG. 1 is a schematic view for showing a network configuration of the conventional ADSL.
Referring to FIG. 1, the ADSL network stipulated by an ADSL forum comprises an ADSL transmission unit-central (ATU-C) 1 connected to a connection node 7, an ADSL transmission unit-remote (ATU-R) 2 connected to a terminal 8, and splitters 3, 4 respectively connected to a public switched telephone network (PSTN) and a telephone of the subscriber 6. A carrierless AM/PM (CAP) system and a DMT system are representatively known as the modulation/demodulation system of the ADSL network and the DMT system is recommended by the ADSL forum and the American National Standards Institute (ANSI). The DMT system is a kind of the multi-subcarrier transmission system that the inputted signal is divided and transmitted onto the 256 subcarriers. That is, the inputted signal source is divided into partial signals αn of n numbers at a transmitting-end according to the following mathematical expression (1), and then the transmission signal X[n] can be expressed as the sum of product of the partial signals αn and an orthogonal function φn corresponding to the subcarrier.X[n]=α0φ0[n]+α1φ1[n]+α2φ2[n]+ . . . +αn−1φn−1[n]  (1) 
wherein the orthogonal function φk[n]=exp(jk·2π/N·n).
At that time, for utilizing a normalized coefficient at a transformation and an inverse transformation, the transmission signal X[n] is expressed as the following mathematical expression (2) in the case that 1√N·ak is used instead of the αn.                               X          ⁡                      [            n            ]                          =                              ∑                          k              =              0                                      n              -              1                                ⁢                                    1                              N                                      ·                          a              k                        ·                          exp              ⁡                              (                                  j                  ⁢                                                                           ⁢                                      k                    ·                                                                  2                        ⁢                                                                                                   ⁢                        π                                            N                                        ·                    n                                                  )                                                                        (        2        )            
Meanwhile, the ak is calculated from the received X[n] at a receiving-end as the following mathematical expression (3).                               a          k                =                              1                          N                                ⁢                                    ∑                              n                =                                  <                  N                  >                                                      ⁢                                          X                ⁡                                  [                  n                  ]                                            ·                              exp                ⁡                                  (                                                            -                      j                                        ⁢                                                                                   ⁢                                          k                      ·                                                                        2                          ⁢                                                                                                           ⁢                          π                                                N                                            ·                      n                                                        )                                                                                        (        3        )            
The ADSL is a high speed data link having one ak stream of the transmitter and one ak stream of the receiver. The low speed data and the high speed data are packaged and transmitted through such data links after the low and the high speed data are allocated to a record spaces in the identical superframe or the packet. Hence, the time delay or the service sorting function is demanded for forming a large frame.
As for the frame and format of the data transmitted through the ADSL link, the ADSL forum suggests a packet mode method and a multi-frame mode method. The packet communication mode is divided into a point-to-point protocol (PPP), a protocol data unit (PDU) packet mode and an asynchronous transfer mode (ATM) frame user to network interface (UNI) packet mode. In the multi-frame mode as shown in the following table 1, a downstream ATM signal of below 6 Mbps and an duplex signal of below 640 kbps are mapped onto one data frame, and then one superframe is formed by gathering 68 data frames.
TABLE 1channeltransmission abilityaccommodation serviceAS0 . . . AS3downstream signal ofVOD,channelbelow 6 Mbpshigh speed one-way ATMLS0 . . . LS2duplex signal ofduplex ATM, data circuit,channelbelow 640 kbpsISDN (2 B + D)
The ATU-C 1 interfaces with the connection node 7 by the limited transmission speed of 2 Mb/s or 155 Mb/s and shares such interface with all the ATU-R which the ATU-C 1 belongs to. Therefore, the traffic bottleneck may occur such as other subscribers use very low traffic or wait for the completion of the high speed data transmission when one subscriber transmits a mass storage file or dynamic picture information.